Gas turbine engines are commonly used to propel aircraft. The efficiency and performance of the gas turbine engine may vary according to the form of the engine and the flight parameters of the aircraft. In general, however, the operating efficiency of conventional gas turbine engines used in aircraft is not optimized throughout the flight envelope. More specifically, the operating efficiency typically includes several components, such as propulsive efficiency and thermal efficiency, that peak at different power settings. The overall pressure ratio (OPR) of a gas turbine engine can influence when peak propulsive or thermal efficiency is reached. In most conventional aircraft, OPR is fixed and therefore the efficiency components typically peak at different power settings.
More recently, variable pressure ratio engines have been proposed in an effort to better coordinate peak propulsive and thermal efficiencies. For example, United States Patent Application Publication No. 2010/0223903 A1 to Starr, published on Sep. 9, 2010, discloses a variable pressure ratio compressor for a gas turbine engine in which rear stages of the compressor may be selectively engaged to increase the pressure ratio. The engine includes an air bypass and a valve controlling access to the bypass. When the rear stages are engaged, the valve is closed so that air flow is directed through the rear stages. Alternatively, when the rear stages are disengaged, the valve is opened to divert air flow around the rear stages. Significantly, the Starr '903 application discloses a clutch for selectively engaging the rear stages that is positioned between the forward and rear compressor stages, which is proximate the combustor. Consequently, the clutch is exposed to temperatures as high as approximately 1000° F., which necessitates the use of high temperature clutch components, thereby increasing the cost of the compressor. Additionally, only a limited space is available at that location, and therefore the assembly proposed in the Starr '903 application appears impractical.